You will need to know your Steam ID for many applications such as plugins, Rust RCON tools, and certain admin commands. 

It's very simple and quick to find. First, go to your steam profile page and within the URL bar, copy your steam community profile URL, from there, insert it into the tool located here: steamid.io.

Steamid.io shows information about your steam account and other user's accounts based on the steam community link.

Information is given such as the users SteamID, SteamID3, Steam64 ID, and more. All of this information may be referenced by commands in other steam games as well so this is a useful link to bookmark. Rust allows you to either use a user's name or a SteamID to ban, kick, or modify user permissions. Permissions groups are further customizable with OxideMod Permissions.